1. Field of the Invention
The invention relates to a method of reducing a registration error and, more particularly to a method of reducing a registration error in an exposure step of a process for manufacturing a semiconductor device.
2. Description of the Background Art
As a pattern of a semiconductor device is miniaturized, the demand for registration accuracy in a photolithography step is increasing. For example, the design rule which refers to the minimum size in the design of a 64MDRAM (Dynamic Random Access Memory) is approximately 0.30-0.35 .mu.m. The registration accuracy required for this design rule is 0.08 .mu.m or less.
FIG. 3 is a perspective view illustrating a conventional exposure apparatus. Referring to FIG. 3, the conventional exposure apparatus is provided with a wafer 101 on a wafer stage 100 which is movable in the X-Y directions. A demagnification projection lens 103 is provided above wafer 101, and a reticle 104 with a patterned layer is placed above demagnification projection lens 103. Using the exposure apparatus of above described structure, a pattern corresponding to the pattern image of reticle 104 is formed on the surface of wafer 101. A shot 102 having one pattern which corresponds to the pattern image of reticle 104 is formed on wafer 101 by an exposure, and a plurality of shots 102 are formed on wafer 101 by repeating the exposure.
An alignment at the time of exposure by the conventional exposure apparatus is performed by moving wafer stage 100 according to an array of shots 102 in the layer pattern (not shown) formed on wafer 101 in the previous step. A general method conventionally used for recognizing the array of shots 102 is EGA (Enhanced Global Alignment). According to this EGA method, the state of placement of some of those shots 102 on wafer 101 is determined by measuring the positions of the marks for registration (alignment marks) provided in the shots 102. The array state of all the remaining shots 102 is accordingly examined. Based on the result, an alignment exposure is executed.
Conventionally, after an exposure according to EGA, an amount of displacement of shot 102 was measured by an alignment inspection device. The amount of displacement means an amount of shift (offset) in the X and Y directions as shown in FIG. 4. For measuring the amount of displacement, marks for inspection of misalignment (not shown) are first arranged in four corners of shot 102. Then, a relative amount of displacement between the first misalignment inspection marks of the first layer pattern formed in the previous step and the second misalignment inspection marks of the second resist pattern formed on the second layer on the first layer pattern is image-processed by a CCD camera, whereby the amount of displacement is calculated. An average value of the amounts of displacement at the four corners of shot 102 was conventionally used as an amount of displacement of shot 102.
If all the shots 102 are determined to be uniformly shifted in the same direction as a result of above inspection by the conventional alignment inspection device, it is understood that a correction for the offset amount was necessary beforehand in the exposure step according to EGA. That is, EGA requires a correction of the amount measured in EGA method in accordance with the positions of the marks for registration (alignment marks) or the production error of a reticle. Feeding back this correction value into an exposure condition setting file within the exposure apparatus, an offset error could be zero when the same exposure step as above will be next performed.
With reference to FIG. 5, conventional ways of calculating and feeding back a correction value will be described in the following.
Referring to FIG. 5, a first layer is formed on a semiconductor substrate (not shown) in S1 (Step 1). In S2, a first resist is applied to the first layer. Next, the applied first resist is exposed by a first exposure apparatus in S3. In S4, a first resist pattern is then formed by developing the exposed first resist. In S5, the first layer is patterned by etching using the first resist pattern thereon as a mask, thereby forming a first layer pattern. Next, a second layer is formed on the first layer pattern in S6. A second resist is then applied to the second layer in S7. In S8, the second resist is exposed by a second exposure apparatus. In S9, a second resist pattern is formed by developing the exposed second resist.
Subsequently, amounts of displacement between the first layer pattern and the second resist pattern are measured at four points (in four corners of a shot) in S10. A determination is made in S11 whether at least one of the four measured amounts of displacement is a prescribed value or more. If all the amounts of displacement are less than the prescribed value, next in S12, a second layer pattern is formed by etching the second layer using the second resist pattern thereon as a mask.
On the other hand, if at least one of the amounts of displacement is determined to be the prescribed value or more, a correction value is calculated and a second resist is formed again. The correction value is determined by firstly calculating an average value of the four measured amounts of displacement as shown in S15. Secondly the determined correction value is fed back to the second exposure apparatus in S8. After above steps, a second resist is formed again. According to the re-formation step of the second resist, the second resist pattern is first removed in S14, then the new second resist is applied again to the second layer in S7.
In the conventional method described above, if an amount of displacement between the first layer pattern and the second resist pattern is a prescribed value or more, a correction value is calculated and the value is fed back to the second exposure apparatus, then the defective second resist pattern is removed and a second resist pattern is newly formed by applying a new second resist to the second layer.
According to the conventional method for exposing a plurality of layers, in some cases an exposure apparatus (first exposure apparatus) employed for a pattern formation (first resist pattern formation) in the previous step is different from an exposure apparatus (second exposure apparatus) employed for a pattern formation (second resist pattern formation) in the following step.
Under above described condition in which a plurality of different exposure apparatuses (steppers) are alternately used, shapes of shots produced by respective exposure apparatuses may be different from each other due to the difference of lens distortion or the like between respective exposure apparatuses. FIG. 6 is a schematic illustration of an exemplary shape of a shot when different exposure apparatuses are employed for the first layer and the second layer. Referring to FIG. 6, the shape of the shot of a first layer pattern 1 is approximately rectangular, while the shape of the shot of a second resist pattern 2 is trapezoidal. As a result, when amounts of displacement between the first layer pattern 1 and the second resist pattern 2 are measured at four measuring points 3 according to S10 of FIG. 5, only an amount of displacement "a" in the X direction at the measuring point in the upper right corner is measured. This result also occurs even when a single exposure apparatus is employed, due to the change of an exposure illumination condition.
According to the conventional method of calculating a correction value shown in S15 of FIG. 5, an average value (0+0+0+a).times.1/4=a/4 of the four amounts of displacement at the four corners of the shot is computed as a correction value. The correction value is fed back to the second exposure apparatus as an offset correction value upon the next exposure. The result of an alignment inspection after the exposure by the second exposure apparatus with the correction value fed back is as shown in FIG. 7. An average value of the amounts of displacement at the four corners of the shot is (-a/4-a/4-a/4+3a/4).times.1/4=0. However, amounts of displacement at respective measuring points in the shot are -a/4, -a/4, -a/4, +3a/4, which means that there is left a point with an amount of displacement of at most 3a/4 in absolute value. If any of the amounts of displacement has such a larger value, various problems such as an electrical short, caused, for example, by a shift of a pattern of a contact hole, will occur. Such problems have become serious with the increasing miniaturization of a semiconductor pattern.